This invention relates to video display systems and, more particularly, to a video display system and scanning method employing propagating stress waves and light emitting means for scanning a viewing plane in orthogonal scanning directions.
In a typical video display system such as that employed in television receivers, a raster scan technique is employed and a beam of electrons is scanned horizontally and vertically through deflection of the electron beam. As the electron beam is scanned, the beam is modulated by a video signal so as to vary the intensity of the beam proportionately to the amplitude of the video signal. The scanned and video modulated electron beam strikes a phosphor coating on a screen element and excites the phosphor in relation to its intensity. The scanning speed is such that a viewer sees an entire image made up of light and dark areas on the screen element without being aware of the scanning which transpires.
Electron beam scanning techniques typically employ evacuated cathode-ray tubes which may be quite large and cumbersome because of the electron beam deflection requirements. The cathode-ray tubes are usually quite expensive both because of the size requirements and because of the internal complexity thereof. Accordingly, numerous attempts have been made to simplify such display systems.
One attempt to simplify video display systems involves the use of solid state devices and what may be termed electro-optical scanning techniques. For example, it has been suggested that scanning may be accomplished in a video display by propagating accoustic energy along a strain responsive semiconductor material in orthogonal directions and by illuminating the entire back surface of the material with intensity modulated light. Stress waves propagating both vertically and horizontally in the semiconductor material selectively modify the light waves passing through the material and a scanning function of sorts is accomplished by an increase in the percentage of light transmitted through the semiconductor material in the region of the intersection of the vertically and horizontally propagating waves.
The propagation velocity of the stress waves in homogenous semiconductor materials is typically quite high and is usually constant independently of the direction of propagation in the material. Therefore, if the scanning period, i.e., the period between successive scanning synchronization signals, is about the same in both the horizontal and vertical directions and is relatively short, this technique may be utilized in a video display device of relatively manageable dimensions. However, in a raster scan system the vertical scanning period is several hundred times as long as the horizontal scanning period. Employing this suggested scanning technique in a raster scan display employing a homogenous stress wave propagating material therefore is impractical, for common standards. Moreover, the selection of the stress wave propagating material is cricital since it must exhibit appropriate light modifying characteristics only at the intersection of two orthogonal stress waves.